Interrupts are increasingly irregular inputs to a digital processor that cause high-overhead context switches whereby the processor halts its current program execution, saves its current context or status, services the interrupt, restores the interrupted context or status and resumes execution of the interrupted program. Examples of interrupts are input/output events that are not within the control of the digital processor to predict or to handle as anything other than an exception. Such exception handling is referred to in the computer world as a context switch. A context switch carries with it an especially high overhead in modern processors, which are equipped with instruction pre-fetch queues and look-ahead logic and instruction pipelines and caches that permit high speed computation. Intermediate results of processor operations must be discarded, e.g. by flushing the instruction pre-fetch cache, and much time is wasted in traditional response to an interrupt. Because interrupt processing is controlled by the digital processor hardware, its priority in time is secondary to the regular, streamlined code and data processing performed by the processor.
The trend is toward increased speed and functionality in computers such as personal computers (PCs), workstations, network servers and the like. But part of the increased speed and functionality is an increase in input/output (I/O) bandwidth and variety. Inputs to computers include keyboards, mouses, joysticks, video cards, musical instrument digital interfaces (MIDIs), compact disc read-only memory devices (CD-ROMs), small computer systems interface (SCSIs), disc drives, zip drives, fax-modems, e-mail and Internet downloads. All such inputs generate hardware interrupts to the processor. All such interrupts to a conventional processor requires a context switch in which the processor halts its current instruction flow, flushes the intermediate results of instruction pre-fetched macro-code execution, saves its address counters and status registers, and performs a branch to unrelated interrupt service routine code. After each hardware interrupt, the processor restores its status registers and address counters and resumes what it was doing. Such hardware interrupts are many—a busy mouse may generate hundreds of interrupts per second. Disc drives and video cards may generate thousands of interrupts per second, each. A single Internet application, while running on your personal computer, may generate tens of thousands of interrupts per second. The context switching overhead adversely impacts the performance of conventional processors each and every time one of these hardware interrupts occurs.
A video card generates a relatively low volume of interrupts at fairly regular intervals, e.g. when the video cache expires. A mouse or a joystick generates a high volume of interrupts at fairly regular intervals, at least while the mouse or joystick is in use. A disc drive generates a relatively high volume of interrupts at irregular intervals. A modem that is being used in a network application similarly generates a relatively high volume of interrupts, also at irregular intervals. Because many network applications such as downloading files or page swapping are themselves disc-intensive, the pace of hardware interrupts from disc drives and modems in network applications are the most difficult to predict. Accordingly, network applications represent a significant challenge in the design of computers that are flexible and responsive to hardware interrupt servicing.